Apparatus and method for controlling doping

ABSTRACT

An apparatus and method, the apparatus comprising: at least one charged substrate (3); a channel of two dimensional material (5); and at least one floating electrode (7A-C) wherein the floating electrode comprises a first area (10A-C) adjacent the at least one charged substrate, a second area (11A-C) adjacent the channel of two dimensional material and a conductive interconnection (9A-C) between the first area and the second area wherein the first area is larger than the second area and wherein the at least one floating electrode is arranged to control the level of doping within the channel of two dimensional material.

TECHNOLOGICAL FIELD

Examples of the disclosure relate to an apparatus and method forcontrolling doping. In particular, examples of the disclosure relate toan apparatus and method for controlling doping in two dimensionalmaterials such as graphene.

BACKGROUND

Two dimensional materials such as graphene may be used in electronicdevices. It is useful to be able to control the doping profiles of suchmaterials.

BRIEF SUMMARY

According to various, but not necessarily all, examples of thedisclosure, there may be provided an apparatus comprising: at least onecharged substrate; a channel of two dimensional material; and at leastone floating electrode wherein the floating electrode comprises a firstarea adjacent the at least one charged substrate, a second area adjacentthe channel of two dimensional material and a conductive interconnectionbetween the first area and the second area wherein the first area islarger than the second area and wherein the at least one floatingelectrode is arranged to control the level of doping within the channelof two dimensional material.

In some examples the apparatus may comprise a plurality of floatingelectrodes. In some examples different floating electrodes may havedifferent first areas. In some examples different floating electrodeshave different second areas. In some examples the different floatingelectrodes are provided adjacent to different portions of the channel oftwo dimensional material to enable different levels of doping to beprovided in different portions of the channel of two dimensionalmaterial.

In some examples the doping within the two dimensional material may bedependent upon an electric field provided by the second area of thefloating electrode.

In some examples, for each floating electrode, a charged substrate andthe first area of the floating electrode may form a first capacitorhaving a first electric field dependent upon the charge on the substrateand wherein the first electric field causes, at the second area of thefloating electrode, a second electric field that is dependent upon thefirst electric field amplified by a ratio of the first area to thesecond area.

In some examples at least one floating electrode may be provided on afirst side of a charged substrate and at least one floating electrodemay be provided on a second side of the charged substrate.

In some examples an insulating material may be provided between thechannel of two dimensional material and the second area of the floatingelectrode.

In some examples the channel of two dimensional material may be providedon a charged substrate. The first area of the floating electrodes mayoverlie a first area of the charged substrate wherein the channel of thetwo dimensional material does not extend over the first area. Aninsulating material may be provided between the charged substrate andthe channel of two dimensional material.

In some examples the channel of two dimensional material may comprisegraphene.

In some examples the at least one charged substrate may comprise atleast one of; a ferroelectric material, a piezoelectric material, apyroelectric material or any other suitable material.

In some examples the apparatus may further comprise a controllerconfigured to control the charge on the at least one charged substrate.

According to various, but not necessarily all, examples of thedisclosure, there may be provided a method comprising: providing atleast one charged substrate; providing a channel of two dimensionalmaterial; and controlling the level of doping within the channel of twodimensional material by providing at least one floating electrodewherein each floating electrode comprises a first area adjacent to theat least one charged substrate, a second area adjacent the channel oftwo dimensional material and a conductive interconnection between thefirst area and the second area wherein the first area is larger than thesecond area.

In some examples the method may further comprise providing a pluralityof floating electrodes. In some examples different floating electrodesmay have different first areas. In some examples different floatingelectrodes may have different second areas. The different floatingelectrodes may be provided adjacent to different portions of the channelof two dimensional material to enable different levels of doping to beprovided in different portions of the channel of two dimensionalmaterial.

In some examples the doping within the two dimensional material may bedependent upon an electric field provided by the second area of thefloating electrode.

In some examples for each floating electrode a charged substrate and thefirst area of the floating electrode may form a first capacitor having afirst electric field dependent upon the charge on the substrate andwherein the first electric field causes, at the second area of thefloating electrode, a second electric field that is dependent upon thefirst electric field amplified by a ratio of the first area to thesecond area.

In some examples the method may further comprise providing at least onefloating electrode on a first side of a charged substrate and providingat least one floating electrode on a second side of the chargedsubstrate.

In some examples the method may further comprise providing an insulatingmaterial between the channel of two dimensional material and the secondarea of the floating electrode.

In some examples the method may further comprise providing the channelof two dimensional material on a charged substrate. The first area ofthe floating electrodes may overlie a first area of the chargedsubstrate wherein the channel of the two dimensional material does notextend over the first area. The method may also comprise providing aninsulating material is provided between the charged substrate and thechannel of two dimensional material.

In some examples the channel of two dimensional material may comprisegraphene.

In some examples the at least one charged substrate may comprise atleast one of; a ferroelectric material, a piezoelectric material, apyroelectric material or any other suitable material.

In some examples the method may further comprise controlling the chargeon the at least one charged substrate.

According to various, but not necessarily all, examples of thedisclosure there may be provided examples as claimed in the appendedclaims.

BRIEF DESCRIPTION

For a better understanding of various examples that are useful forunderstanding the detailed description, reference will now be made byway of example only to the accompanying drawings in which:

FIG. 1 illustrates an apparatus;

FIG. 2 illustrates a perspective view of an apparatus:

FIG. 3 illustrates a cross section through the apparatus of FIG. 2 and acorresponding doping profile;

FIG. 4 illustrates an equivalent circuit diagram for an apparatus;

FIG. 5 illustrates a cross section through another apparatus and acorresponding doping profile;

FIGS. 6A to 6D illustrate an example apparatus and results obtained withexample apparatus;

FIGS. 7A to 7C illustrate an example apparatus and results obtained withexample apparatus; and

FIG. 8 illustrates a method according to examples of the disclosure.

DETAILED DESCRIPTION

The Figures illustrate an apparatus 1 comprising: at least one chargedsubstrate 3; a channel of two dimensional material 5; and at least onefloating electrode 7 wherein the floating electrode 7 comprises a firstarea 10 adjacent the at least one charged substrate 3, a second area 11adjacent the channel of two dimensional material 5 and a conductiveinterconnection 9 between the first area 10 and the second area 11wherein the first area 10 is larger than the second area 11 and whereinthe at least one floating electrode 7 is arranged to control the levelof doping within the channel of two dimensional material 5.

The apparatus 1 may be for controlling a doping pattern within a twodimensional material.

FIG. 1 schematically illustrates an apparatus 1 according to examples ofthe disclosure. The apparatus 1 comprises at least one charged substrate3, a channel of two dimensional material 5, and at least one floatingelectrode 7. Only one floating electrode 7 is illustrated in FIG. 1. Itis to be appreciated that a plurality of floating electrodes 7 may beprovided in other examples of the disclosure.

The at least one charged substrate 3 may comprise any material which maybe configured to have a non-zero polarization. The charged substrate maycomprise bound charges. For instance the charged substrate 3 couldcomprise at least one of; a ferroelectric material, a piezoelectricmaterial, a pyroelectric material or any other suitable material whichmay be engineered to have a non-zero polarization.

In some examples the charge distribution across the at least one chargedsubstrate 3 may be uniform across the surface of the substrate.

In some examples a control signal 2 may be provided to control thecharge of the charged substrate 3. In some examples the charge on thesubstrate 3 may be dependent upon a parameters such as the temperatureor deformation of the at least one charged substrate 3 or any otherparameter.

The channel of two dimensional material 5 may comprise any twodimensional material which has a resistance which may be tuned by thefield effect. For example the channel of two dimensional material 5 maycomprise a monolayer of graphene or any other suitable material.

In some examples the two dimensional material 5 may be provided on acharged substrate. In such examples the channel of two dimensionalmaterial 5 may comprise a material which may be grown on or deposited onthe charged substrate 3.

The floating electrode 7 may comprise any conductive material such asmetal, semiconductor, two dimensional material, ionic-liquid, ionic gelor any other suitable material. In some examples the floating electrode7 may comprise graphene or indium tin oxide or any other suitablematerial.

In some examples the floating electrode 7 may be deformable and/ortransparent.

The floating electrode 7 comprises a first area 10, a second area 11 anda conductive interconnection 9. The first area 10 is provided adjacentto the at least one charged substrate 3. The second area 11 is providedadjacent to the channel of two dimensional material 5. The conductiveinterconnection 9 is provided between the first area 10 and the secondarea 11. The floating electrode 7 may be formed as separatedinterconnected components or as a single integral component, forexample, as a patterned layer of the same material.

The first area 10 may be larger than the second area 11. The sizedifference of the first area 10 compared to the second area 11 mayenable the floating electrode 7 to amplify an electrostatic voltage atthe first area 10 to a larger electrostatic voltage at the second area11. The electrostatic voltage at the first area 10 is a result of chargeon the charged substrate 3. The electrostatic voltage at the second area11 is dependent upon the electrostatic voltage at the first area 10amplified by a ratio of the first area 10 to the second area 11.

In some examples the floating electrode 7 may be electrically isolatedor electrically isolatable. That is, the floating electrode 7 may be anelectrode that may be permanently electrically isolated or switched tobecome electrically isolated. The isolation ensures that the floatingelectrode 7 is a closed electrical circuit that conserves charge. Thereis no direct current path between the floating electrode 7 and thechannel of two dimensional material 5.

FIG. 2 illustrates a perspective view of an apparatus 1 according toexamples of the disclosure. The apparatus 1 comprises at least onecharged substrate 3, a channel of two dimensional material 5, and aplurality of floating electrodes 7 which may be as described above.Corresponding reference numerals are used for corresponding features.

In the example apparatus 1 of FIG. 2 a single charged substrate 3 isprovided. Each of the floating electrodes 7 has an area which isprovided adjacent to the charged substrate 3. The charged substrate 3may underlie all of the floating electrodes 7 and the channel of twodimensional material 5.

In the example of FIG. 2 three floating electrodes 7A, 7B and 7C areprovided. It is to be appreciated that any number of floating electrodes7 may be provided in other examples of the disclosure. Each of thefloating electrodes 7A, 7B and 7C comprises a first area 10A, 10B, 10C,a second area 11A, 11B, 11C and a conductive interconnection 9A, 9B, 9C.

Where a plurality of floating electrodes 7 are provided, differentfloating electrodes 7 may have different sized first areas 10 and/ordifferent sized second areas 11. In the example of FIG. 2 each of thefloating electrodes 7A, 7B, 7C have different sized first areas 10A,10B, 10C but the same sized second areas 11A, 11B, 11C. It is to beappreciated that other arrangements could be used in other examples.

The channel of two dimensional material 5 is formed on top of thecharged substrate 3. The channel of two dimensional material 5 isprovided between a source 21 and a drain 23. In some examples thechannel of two dimensional material 5 and the source 21 and the drain 23may be provided by a layer of graphene or any other suitable material.Conductive terminals may be applied to the source 21 and, separately, tothe drain 23.

An insulating material 25 is provided over the channel of twodimensional material 5. The insulating material 25 may be providedbetween the channel of two dimensional material 5 and the second area 11of the floating electrode 7. The insulating material 25 may preventelectrical connection between the floating electrode 7 and the channelof two dimensional material 5. The insulation material 25 may comprise adielectric material or any other suitable material.

The floating electrodes 7 may be configured to control the doping withinthe channel of two dimensional material 5. Each of the floatingelectrodes 7A, 7B, 7C comprises a first area 10A, 10B, 10C providedadjacent to the charged substrate 3. In the example of FIG. 2 each ofthe different floating electrodes 7A, 7B, 7C has a different sized firstarea 10A, 10B, 10C. The electrostatic voltage at each of the first areas10 is a dependent upon the size of the overlap between the first area 10and the charged substrate 3. Therefore in the example of FIG. 2 each ofthe floating electrodes 7A, 7B, 7C will have different electrostaticvoltages at the respective first areas 10A, 10B, 10C.

Each of the floating electrodes 7A, 7B, 7C also comprises a second area11A, 11B, 11C provided adjacent to the channel of two dimensionalmaterial 5. Each of the second areas 11A, 11B, 11C is connected to thefirst areas 10A, 10B, 10C by a corresponding conductive interconnect 9A,9B, 9C.

In the example of FIG. 2 each of the second areas 11A, 11B, 11C is thesame size so that each of the floating electrodes 7A, 7B, 7C has thesame overlap with the channel of two dimensional material 5. As each ofthe floating electrodes 7A, 7B, 7C have different electrostatic voltagesat the respective first areas 10A, 10B, 10C this means that, in theexample of FIG. 2 each of the floating electrodes 7A, 7B, 7C will alsohave different electrostatic voltages at the respective second areas11A, 11B, 11C. This creates different electric fields at differentpoints along the channel of two dimensional material 5 which providesdifferent levels of doping at different positions within the channel oftwo dimensional material 5.

FIG. 3 illustrates a cross section through the apparatus 1 of FIG. 2 anda doping profile along the channel of two dimensional material 5.

FIG. 3 is a cross section through the line X-Y. Corresponding referencenumerals are used for corresponding features. In this example the twodimensional material is intrinsically n-type. The charged substrate 3produces a negative charge density on the surface of the substrate 3.The electric field at the second areas 11A, 11B, 11C of the floatingelectrodes 7A, 7B, 7C causes the two dimensional material to become morep-type.

In the example apparatus of FIGS. 2 and 3 the first areas 10A, 10B, 10Cof the electrodes increase in size along the length of the channel oftwo dimensional material 5. This increases the size of the electricfield along the length of the channel of two dimensional material 5 andcauses the doping to become increasingly p-type.

FIG. 4 illustrates an equivalent circuit diagram for the floatingelectrodes 7 and the charged substrate 3 of the apparatus 1 of FIGS. 1to 3.

Each floating electrode 7 enables a combination of two capacitors inseries to be formed. The charged substrate 3 and the first area 10 ofthe floating electrode 7 form a first capacitor C_(1P). The firstcapacitor C_(1P) has an effective area A_(1P) corresponding to the firstarea 10 of the floating electrode 7. The first capacitor C_(1P) stores acharge Q_(1P) over the area A₁ and develops a voltage V_(1G).

The channel of two dimensional material 5 and the second area 11 of thefloating electrode 7 form a second capacitor C_(1G). The secondcapacitor C_(1G) has an effective area A_(1G) corresponding to thesecond area 11 of the floating electrode 7. The second capacitor C_(1G)stores a charge Q_(1G) over the area A_(1G) and develops a voltageV_(1G).

As the bias of the channel of two dimensional material 5 is usuallyquite low (that is less than 1 V) the two dimensional material 5 of thesecond capacitor C_(1G) can be treated as ground.

As the capacitors C_(1P) and C_(1G) are in series, for a givenpolarization P of the charged substrate 3, the charge on both capacitorsC_(1P), C_(1G) must be the same. That is:Q _(1P) =Q _(1G)

Therefore the top gate potential applied to channel of two dimensionalmaterial 5 under the second area 11 of the floating electrode 7 is:

$V_{1\; G} = {\frac{Q_{1\; G}}{C_{1\; G}} = \frac{Q_{1\; P}}{C_{1\; G}}}$

As C_(1G) is constant, the voltage V_(1G) is proportional to the chargeQ_(1P) and thus proportional to the polarization P of the chargedsubstrate 3 and the first area 10.Q _(1P) =P*A _(1P)

The doping profile may be modulated by using different floatingelectrodes 7 to apply different top-gate voltages to different regionsof the channel of two dimensional material 5. The different top-gatevoltages may be provided at zero energy cost by the charged substrate 3.

The different top-gate voltages may be controlled by the respectiveareas 10, 11 of the floating electrode 7.

As the second voltage V_(1G) scales with the capacitance ratioC_(1P)/C_(1G) different second voltages V_(1G) may be provided by havingdifferent floating electrodes 7 with different capacitance ratiosC_(1P)/C_(1G). In some examples it may be desirable for C_(1P) to belarger than C_(1G). This may be achieved by making the first area 10larger than the second area 11.

Therefore the charged substrate 3 and the first area 10 of the floatingelectrode 7 form a first capacitor C_(1P) having a first electric fielddependent upon a polarization of the charged substrate 3. The firstelectric field causes, at the second area 11 of the floating electrode,a second electric field that is dependent upon the first electric fieldamplified by a ratio of the first area 10 to the second area 11.

It is to be appreciated that a third capacitor C₀ may be formed directlyby the channel of two dimensional material 5 and the charged substrate3. This may affect the doping level within the channel of twodimensional material 5. However this doping effect is an offset thatapplies to the whole channel of two dimensional material 5 and does notcontribute to any modulation doping. The direct effect of the chargedsubstrate 3 on channel of two dimensional material 5 is significantlylower than the effect of the effect of the charged substrate 3 on thefirst area 10 of the floating electrode 7. This allows the capacitor C₀to be disregarded in the above explanation.

In some examples an insulating material may be provided between thecharged substrate 3 and the channel of two dimensional material 5. Thismay remove the capacitor C₀ by detaching the channel of two dimensionalmaterial 5 from the charged substrate 3.

In the example of FIGS. 2 and 3 the floating electrodes 7 have differentsized first areas 10 but same sized second areas 11. It is to beappreciated that different sized and shaped floating electrodes 7 may beused to obtain different doping profiles. There are virtually noboundaries to the complexity and shape that can be obtained for thedoping profile of the two dimensional material. The only limitation onthe doping profile is the area available on the charged substrate 3.

In some examples of the disclosure all of the floating electrodes 7 maybe manufactured in a single step. This may be achieved regardless of howmany floating electrodes 7 are needed, the sizes of the floatingelectrodes 7 and the variation in shapes of the different floatingelectrodes 7.

FIG. 5 illustrates a cross section through another apparatus 1 and acorresponding doping profile. The apparatus 1 comprises a chargedsubstrate 3 and a channel of two dimensional material 5 which extendsbetween a source 21 and a drain 23 which may be as described above. Theapparatus 1 also comprises a first floating electrode 7D and a secondfloating electrode 7E.

The floating electrodes 7D and 7E of the apparatus of FIG. 5 may be asdescribed above and may each comprise a first area 10D, 10E adjacent tothe charged substrate 3 and a second area 11D, 11E adjacent to thechannel of two dimensional material 9 and a conductive interconnect 9D,9E as described above. As FIG. 5 is a cross section only portions of therespective second areas 11D, 11E are illustrated in FIG. 5.

In the example of FIG. 5 the first floating electrode 7D is provided ona first side 51 of the charged substrate 3 and the second floatingelectrode 7E is provided on a second side 53 of the charged substrate 3.A layer of insulating material 25 is also provided between the channelof two dimensional material 5 and each of the floating electrodes 7D,7E.

In the example of FIG. 5 the second area 11D of the first floatingelectrode 7D is the same as the second area 11E of the second floatingelectrode 7E. The first floating electrode 7D has a larger first area10D than the second floating electrode 7E. Although the respective firstareas 10D, 10E are not shown in FIG. 5 they can be deduced from therespective doping levels.

In the example of FIG. 5 the second area 11D of the first electrode 7Doverlaps with the second area 11E of the second electrode 7E. Thisprovides a double gated region 55 within the channel of two dimensionalmaterial 5. This provides the strongest electric field and therefore thehighest doping level within the double gated region 55.

In the example of FIG. 5 one floating electrode 7 is provided on eitherside 51, 53 of the substrate. It is to be appreciated that any number offloating electrodes 7 may be provided on either side 51, 53 of thecharged substrate 3.

Providing floating electrodes 7 on either side of the substrate mayprovide several advantages. It may reduce the number of floatingelectrodes 7 which are needed as different doping levels may be achievedby overlapping respective floating electrodes 7. It may also remove thethird capacitor C₀ which may make the apparatus 1 simpler.

FIGS. 6A to 6D illustrate example devices and results obtained with theexample devices. The example apparatus 1 comprises a channel of twodimensional material 5, a charged substrate 3 and at least one floatingelectrode 7 as described above.

FIG. 6A shows a first example apparatus 1 and FIG. 6C shows a secondexample apparatus 1. In both of the example apparatus the channel of twodimensional material 5 comprises graphene. In both of the examples apyroelectric material, z-cut LiNbO₃, was used as the charged substrate3.

The two example apparatus 1 have different sized floating electrodes 7.In the example apparatus 1 of FIG. 6A the first area 10 of the floatingelectrode A=10⁻⁴ cm². In the example apparatus 1 of FIG. 6C the firstarea 10 of the floating electrode 3A=3*10⁻⁴ cm².

FIGS. 6B and 6D are plots of the electrical measurements obtained withthe respective apparatus 1. The electrical measurements were taken at aconstant temperature of 293K. It can be seen that the different floatingelectrodes translated the constant polarization of the charged substrate3 into different negative top-gate voltages. FIG. 6B shows that thisresulted in the apparatus of FIG. 6A retaining some of the originaln-type nature of the graphene. FIG. 6D shows that this also resulted inthe apparatus of FIG. 6C undergoing a larger doping shift leading top-type behaviour. Therefore this shows that different floatingelectrodes 7 can produce different levels of doping.

FIGS. 7A to 7C illustrate another example apparatus 1 and resultsobtained with this example apparatus 1. The example apparatus of FIG. 7Acomprises a charged substrate 3, a channel of two dimensional material 5and two floating electrodes 7F and 7G which may be as described above.In the example of FIG. 7A the first floating electrode 7F has a smallerfirst area 10F than the second floating electrode 7G. The two floatingelectrode 7F, 7G have the same sized second areas 11F, 11G.

The example apparatus 1 may also comprise an external drive which mayenable the charge on the substrate 3 to be controlled. For instance thecharged substrate 3 may comprise a material such as a ferroelectric,piezoelectric or pyroelectric material which has a polarization that canbe varied by means of electric fields, mechanical stress andtemperature, respectively.

In the example of FIG. 7A the apparatus 1 comprises a ferroelectricmaterial in which the polarization is controlled by an electric device.In the example of FIG. 7A the overall polarization of the chargedsubstrate 3 can be switched between positive polarization P+ andnegative polarization P−. The switch in polarization may be achieved byapplying a voltage to a bottom gate connected to the charged substrate3. It is to be appreciated that other materials and means forcontrolling polarization may be used in other examples of thedisclosure.

FIG. 7B shows results obtained when the charged substrate has a negativepolarization P− so that a negative surface charge is developed on thefirst surface 51 of the charged substrate 3.

In FIG. 7B the dot 71 represents the un-gated two dimensional materialand so shows the intrinsic doping of the two dimensional material. Inthis example the intrinsic doping is n-type. The dot 7F represents theregion gated by the first floating electrode 7F which remains n-type.The dot 7G represents the region gated by the second floating electrode7G. The doping in this region is more negatively shifted and becomesp-type.

FIG. 7C shows results obtained when the charged substrate has a positivepolarization P+ so that a positive surface charge is developed on thefirst surface 51 of the charged substrate 3.

In FIG. 7C the dot 71 represents the un-gated two dimensional materialand so shows the intrinsic doping of the two dimensional material. As inthe previous example the intrinsic doping is n-type. The dot 7Frepresents the region gated by the first floating electrode 7F and thedot 7G represents the region gated by the second floating electrode 7G.In this example a highly n-doped region is provided.

Therefore the example apparatus 1 of FIG. 7A can be switched betweendifferent doping patterns by switching the polarization of the chargedsubstrate 3. This may be useful in forming devices which require a p-njunction such as photodetector or other type sensor. The p-n junctionmay be switched on and off by changing the polarization of the chargedsubstrate 3.

In the example of FIGS. 7A to 7C the polarization of the chargedsubstrate 3 is switched between positive polarization P+ and negativepolarization P−. It is to be appreciated that in some examples thedoping profile may be controlled by continuously tuning the charge onthe charged substrate 3. For instance the temperature of a pyroelectricsubstrate may be tuned to tune the polarization of the pyroelectricmaterial.

FIG. 8 illustrates a method of providing an apparatus 1 according toexamples of the disclosure. The method may be used to provide anapparatus 1 as described above. The method comprises providing, at block81, at least one charged substrate 3. The method also comprisesproviding, at block 83, a channel of two dimensional material 5 and atblock 85 the method comprises controlling the level of doping within thechannel of two dimensional material 5. Controlling the level of dopingmay comprise providing at least one floating electrode 7 wherein eachfloating electrode 7 comprises a first area 10 adjacent the at least onecharged substrate 3, a second area 11 adjacent the channel of twodimensional material 5 and a conductive interconnection 9 between thefirst area 10 and the second area 11 wherein the first area 10 is largerthan the second area 11.

Examples of the disclosure provide for an apparatus and method whichallow for the doping profile of a two dimensional material to becontrolled. The apparatus 1 allows for design freedom in the dopingprofile as the shape of the patterns and the level of doping can becontrolled through the sizes of the respective areas 10, 11 of afloating electrode.

The floating electrodes 7 may be fabricated in a single step. This mayenable apparatus 1 with complex doping patterns within a two dimensionalmaterial to be easily fabricated.

The apparatus 1 also allows for the doping profile to be changed bychanging the charge on the charged substrate 3. This may enable certaindevices to be switched between different operating states.

In the above examples the term coupled means operationally coupled. Itis to be appreciated that any number or combination of interveningelements can exist including no intervening elements.

The term “comprise” is used in this document with an inclusive not anexclusive meaning. That is any reference to X comprising Y indicatesthat X may comprise only one Y or may comprise more than one Y. If it isintended to use “comprise” with an exclusive meaning then it will bemade clear in the context by referring to “comprising only one . . . ”or by using “consisting”.

In this brief description, reference has been made to various examples.The description of features or functions in relation to an exampleindicates that those features or functions are present in that example.The use of the term “example” or “for example” or “may” in the textdenotes, whether explicitly stated or not, that such features orfunctions are present in at least the described example, whetherdescribed as an example or not, and that they can be, but are notnecessarily, present in some of or all other examples. Thus “example”,“for example” or “may” refers to a particular instance in a class ofexamples. A property of the instance can be a property of only thatinstance or a property of the class or a property of a sub-class of theclass that includes some but not all of the instances in the class. Itis therefore implicitly disclosed that a features described withreference to one example but not with reference to another example, canwhere possible be used in that other example but does not necessarilyhave to be used in that other example.

Although examples of the disclosure have been described in the precedingparagraphs with reference to various examples, it should be appreciatedthat modifications to the examples given can be made without departingfrom the scope of the invention as claimed.

Features described in the preceding description may be used incombinations other than the combinations explicitly described.

Although functions have been described with reference to certainfeatures, those functions may be performable by other features whetherdescribed or not.

Although features have been described with reference to certainembodiments, those features may also be present in other embodimentswhether described or not.

Whilst endeavoring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

We claim:
 1. An apparatus comprising: at least one charged substrate; achannel of two dimensional material provided between a source electrodeand a drain electrode; and a plurality of electrically isolated floatingelectrodes provided adjacent to different portions of the channel of twodimensional material so that they overlap with the channel oftwo-dimensional material, wherein each electrically isolated floatingelectrode in the plurality of electrically isolated floating electrodescomprises a first area adjacent to the at least one charged substrate, asecond area adjacent to the channel of two dimensional material, and aconductive interconnection between the first area and the second area,wherein the first area is larger than the second area.
 2. The apparatusas claimed in claim 1, wherein the doping within the two dimensionalmaterial is dependent upon an electric field provided by the second areaof the plurality of electrically isolated floating electrodes.
 3. Theapparatus as claimed in claim 1, wherein, for the plurality ofelectrically isolated floating electrodes, a charged substrate and thefirst area of each of the plurality of electrically isolated floatingelectrodes form a first capacitor having a first electric fielddependent upon the charge on the substrate and wherein the firstelectric field causes, at the second area of each of the plurality ofelectrically isolated floating electrodes, a second electric field thatis dependent upon the first electric field amplified by a ratio of thefirst area to the second area.
 4. The apparatus as claimed in claim 1,wherein an insulating material is provided between the channel of twodimensional material and the second area of the plurality ofelectrically isolated floating electrodes.
 5. The apparatus as claimedin claim 1, wherein the channel of two dimensional material is providedon a charged substrate wherein the first area of the plurality ofelectrically isolated floating electrodes overlays a first area of thecharged substrate wherein the channel of the two dimensional materialdoes not extend over the first area.
 6. The apparatus as claimed inclaim 5, wherein an insulating material is provided between the chargedsubstrate and the channel of two dimensional material.
 7. The apparatusas claimed in claim 1, wherein the channel of two dimensional materialcomprises graphene.
 8. The apparatus as claimed in claim 1, wherein theat least one charged substrate comprises at least one of a ferroelectricmaterial, a piezoelectric material, a pyroelectric material, or anyother suitable material.
 9. The apparatus as claimed in claim 1, furthercomprising a controller configured to control the charge on theplurality of electrically isolated floating electrodes.
 10. A methodcomprising: providing at least one charged substrate; providing achannel of two dimensional material between a source electrode and adrain electrode; and controlling the level of doping within the channelof two dimensional material by providing a plurality of electricallyisolated floating electrodes adjacent to different portions of thechannel of two dimensional material so that they overlap with thechannel of two-dimensional material, wherein each electrically isolatedfloating electrode in the plurality of electrically isolated floatingelectrodes comprises a first area adjacent to the at least one chargedsubstrate, a second area adjacent to the channel of two dimensionalmaterial, and a conductive interconnection between the first area andthe second area wherein the first area is larger than the second area.